Mineral dust aerosols play an important role in the climate system. Coupled climate‐aerosol models are an important tool with which to quantify dust fluxes and the associated climate impact. Over the ...last decade or more, numerous models have been developed, both global and regional, but to date, there have been few attempts to compare the performance of these models. In this paper a comparison of five regional atmospheric models with dust modules is made, in terms of their simulation of meteorology, dust emission and transport. The intercomparison focuses on a 3‐day dust event over the Bodélé depression in northern Chad, the world's single most important dust source. Simulations are compared to satellite data and in situ observations from the Bodélé Dust Experiment (BoDEx 2005). Overall, the models reproduce many of the key features of the meteorology and the large dust plumes that occur over the study domain. However, there is at least an order of magnitude range in model estimates of key quantities including dust concentration, dust burden, dust flux, and aerosol optical thickness. As such, there remains considerable uncertainty in model estimates of the dust cycle and its interaction with climate. This paper discusses the issues associated with partitioning various sources of model uncertainty.
The influence of absorbing aerosol on atmospheric conditions in Europe is simulated for a summer and a winter period with a regional model. Depending on the fraction of elemental carbon the effects ...of radiation are determined. Absorbing aerosol increases the average solar heating rate for the summer case by 2.5 × 10−6–4.4 × 10−6 K s−1(20%–46%) within the boundary layer. Due to the heating by absorbing substances an average decrease of the total cloud cover (summer: 1.0%, winter: 0.7%) is found. This semi‐direct radiative effect causes mainly positive forcing near the surface and at the top‐of‐atmosphere (TOA). Significant negative correlations (summer: −0.7, winter: −0.4) between the aerosol optical depth and the direct radiative forcing (DRF) are determined at the surface. At the TOA the DRF pattern is influenced by the surface albedo and the cloud fraction. A general decrease of 2m temperatures is simulated when using absorbing aerosol compared to an aerosol‐free troposphere (summer: −0.14 K, winter: −0.10 K) over land surface.
Key Points
Simulation of European aerosol for two time periods
Computing direct and semi‐direct radiative effects
Semi‐direct radiative effect can be considerable
The Hamburg Aerosol Module version 2.3 (HAM2.3) from the ECHAM6.3‐HAM2.3 global atmosphere‐aerosol model is coupled to the recently developed icosahedral nonhydrostatic ICON‐A (icon‐aes‐1.3.00) ...global atmosphere model to yield the new ICON‐A‐HAM2.3 atmosphere‐aerosol model. The ICON‐A and ECHAM6.3 host models use different dynamical cores, parameterizations of vertical mixing due to sub‐grid scale turbulence, and parameter settings for radiation balance tuning. Here, we study the role of the different host models for simulated aerosol optical thickness (AOT) and evaluate impacts of using HAM2.3 and the ECHAM6‐HAM2.3 two‐moment cloud microphysics scheme on several meteorological variables. Sensitivity runs show that a positive AOT bias over the subtropical oceans is remedied in ICON‐A‐HAM2.3 because of a different default setting of a parameter in the moist convection parameterization of the host models. The global mean AOT is biased low compared to MODIS satellite instrument retrievals in ICON‐A‐HAM2.3 and ECHAM6.3‐HAM2.3, but the bias is larger in ICON‐A‐HAM2.3 because negative AOT biases over the Amazon, the African rain forest, and the northern Indian Ocean are no longer compensated by high biases over the sub‐tropical oceans. ICON‐A‐HAM2.3 shows a moderate improvement with respect to AOT observations at AERONET sites. A multivariable bias score combining biases of several meteorological variables into a single number is larger in ICON‐A‐HAM2.3 compared to standard ICON‐A and standard ECHAM6.3. In the tropics, this multivariable bias is of similar magnitude in ICON‐A‐HAM2.3 and in ECHAM6.3‐HAM2.3. In the extra‐tropics, a smaller multivariable bias is found for ICON‐A‐HAM2.3 than for ECHAM6.3‐HAM2.3.
Plain Language Summary
Aerosols are tiny particles in the air which are either emitted into the atmosphere directly or formed from precursor gases such as sulfur dioxide. Aerosols reflect and absorb solar radiation and affect the radiative properties of clouds. In order to estimate how changing emissions of aerosol precursor gases and aerosols affect the radiation budget of the atmosphere, aerosol models are coupled to global atmosphere models. Here, an aerosol model that has already been part of a well‐established coupled model is coupled to a recently developed atmosphere model. The reasons for differences between the original and the new model are investigated and simulated aerosol optical thickness is evaluated against observations. The aerosol optical thickness over subtropical oceans is lower in the new model, which is in better agreement with estimates from satellite observations. This better agreement is traced back to a parameter setting in the cloud description part in the new model. However, because cancellation of positive and negative biases is reduced in the new model, the global mean aerosol optical thickness is biased lower the new model. A bias score based on several meteorological variables is lower in the new model because of lower biases in the extra‐tropics.
Key Points
The new ICON‐A‐Hamburg Aerosol Module version 2.3 (HAM2.3) global atmosphere‐aerosol model is introduced
The sensitivity of simulated aerosol optical thickness (AOT) to using two different host models but the same aerosol module is investigated
A positive AOT bias over subtropical oceans is remedied in ICON‐A‐HAM2.3 because of a different parameter setting in the host model
On the basis of a new regional dust model system, the sensitivity of radiative forcing to dust aerosol properties and the impact on atmospheric dynamics were investigated. Uncertainties in optical ...properties were related to uncertainties in the complex spectral refractive index of mineral dust. The climatological‐based distribution of desert‐type aerosol in the radiation scheme of the nonhydrostatic regional model LM was replaced by dust optical properties from spectral refractive indices, derived from in situ measurements, remote sensing, bulk measurements, and laboratory experiments, employing Mie theory. The model computes changes in the solar and terrestrial irradiance from a spatially and temporally varying atmospheric dust load for five size classes. A model study of a Saharan dust outbreak in October 2001 was carried out when large amounts of Saharan dust were transported to Europe. The dust optical thickness computed from the simulation results in values of about 0.5 in large regions of the Saharan desert but can be larger than 5.0 near large dust sources (for example, Bodélé depression). During the dust outbreak, the aerosol in the southern Sahara causes a daytime reduction in 2‐m temperature of 3 K in average with differences of 10% depending on used dust optical properties. The simulations indicated that the large variability in radiative properties due to different mixture of clay aggregates in Saharan dust can lead in regional average to differences of up to 48% in net forcing efficiency at top of the atmosphere.
During the SAMUM field campaign in southern Morocco in May and June 2006 density currents generated by evaporative cooling after convective precipitation were frequently observed at the Sahara side ...of the Atlas Mountain chain. The associated strong surface cold‐air outflow during such events has been observed to lead to dust mobilization in the foothills. Here a regional model system is used to simulate a density current case on 3 June 2006 and the subsequent dust emission. The model studies are performed with different parameterization schemes for convection, and with different horizontal model grid resolutions to examine to which extent the model system can be used for reproducing dust emissions in this region. The effect of increasing the horizontal model grid resolution from 14 km to 2.8 km on the strength on the density currents and thus on dust emission is smaller than the differences due to different convection parameterization schemes in this case study. While the results in reproducing the observed density current at the Atlas Mountain varied with different convection parameterizations, the most realistic representation of the density current is obtained at 2.8 km grid resolution at which no parameterization of deep convection is needed.
A new regional model system was developed for simulation of emission, transport, deposition, and radiative effects of Saharan desert aerosol within the framework of the Saharan Mineral Dust ...Experiment (SAMUM). For this the mesoscale meteorological model LM, a dust emission scheme and a transport model were coupled. To test the model performance, two major Saharan dust outbreaks directed to Europe in August and October 2001 are simulated. Comparisons with sounding data and 10‐m wind speeds from north African sites show that the LM provides reliable meteorological fields to describe the emission and near‐source transport of dust. As shown by comparisons with satellite observations, lidar profiles, and Sun photometer measurements at selected stations, the spatiotemporal evolution of the dust plume is reasonably well reproduced by the model. The predicted dust interacts with the LM radiation at solar and thermal wavelengths. Saharan dust causes a negative effect on the net radiative budget at the top of the atmosphere in the source regions and accounts for a reduction in 10‐m wind speeds. Thus it is responsible for a reduction in the dust production of up to about 50% during the October 2001 event.
Mass deposition fluxes of mineral dust to the tropical northeast Atlantic Ocean were determined within this study. In the framework of SOPRAN (Surface Ocean Processes in the Anthropocene), the ...interaction between the atmosphere and the ocean in terms of material exchange were investigated at the Cape Verde atmospheric observatory (CVAO) on the island Sao Vicente for January 2009. Five different methods were applied to estimate the deposition flux, using different meteorological and physical measurements, remote sensing, and regional dust transport simulations. The set of observations comprises micrometeorological measurements with an ultra-sonic anemometer and profile measurements using 2-D anemometers at two different heights, and microphysical measurements of the size-resolved mass concentrations of mineral dust. In addition, the total mass concentration of mineral dust was derived from absorption photometer observations and passive sampling. The regional dust model COSMO-MUSCAT was used for simulations of dust emission and transport, including dry and wet deposition processes. This model was used as it describes the AOD's and mass concentrations realistic compared to the measurements and because it was run for the time period of the measurements. The four observation-based methods yield a monthly average deposition flux of mineral dust of 12–29 ng m−2 s−1. The simulation results come close to the upper range of the measurements with an average value of 47 ng m−2 s−1. It is shown that the mass deposition flux of mineral dust obtained by the combination of micrometeorological (ultra-sonic anemometer) and microphysical measurements (particle mass size distribution of mineral dust) is difficult to compare to modeled mass deposition fluxes when the mineral dust is inhomogeneously distributed over the investigated area.
Simulations of Saharan dust emission, transport, and deposition are performed using new developments of the regional model COSMO‐MUSCAT for the Saharan Mineral Dust Experiment (SAMUM‐1), which took ...place in May–June 2006. Up‐to‐date surface soil data sets developed especially to model dust emissions are used, and a new representation of the dust size distribution is proposed. Compared with previous model studies performed with COSMO‐MUSCAT, the advantage of our approach is that no tuning factor on the erosion threshold is needed for the whole Sahara. The performances and limitations of COSMO‐MUSCAT to model the regional dust cycle are discussed. The spatiotemporal variability of simulated Saharan emissions is evaluated using a backtracking approach to locate dust sources with Meteosat Second Generation (MSG) infrared difference images and using dust observations of North African meteorological stations. The Saharan dust emissions are estimated to be 78 Tg during the studied period. The model dust size distributions agree well with SAMUM‐1 airborne measurements, previous model simulations, and Aerosol Robotic Network (AERONET) inversion products. Our ability to simulate the vertical cross section of a dust plume is also discussed with regard to airborne lidar measurements and former simulations. The horizontal distribution of model‐derived aerosol optical thicknesses (AOT) is compared with Aqua‐MODIS Deep Blue AOT and Ozone Monitoring Instrument (OMI) aerosol indexes. Dry and wet deposition rates are simulated. About 67% of the emitted dust is deposited in the vicinity of the emitted source areas over North Africa and close marine areas, and 33% is transported out of the studied area toward other continents and remote ocean areas.
The influence of volcanic ash on heterogeneous ice nucleation in tropospheric clouds is investigated on the basis of 90 observed cloud cases. The clouds were observed with polarization lidars at the ...two central‐European EARLINET stations Leipzig (51.3°N, 12.4°E) and Maisach (48.2°N, 11.3°E, 25 km northwest of Munich), Germany, in volcanic aerosol layers which originated from the strong eruptions of the Icelandic Eyjafjallajökull volcano in April 2010. Case studies of evolving boundary layer cumuli and long‐lasting free tropospheric cloud events with unusual behavior (mixed‐phase cloud complex, cirrus deck) are discussed. A clear impact of ash is observed. The ice nuclei concentration derived from the lidar observations has been estimated to range from 2–20 per liter in the boundary layer and from 100–300 per liter at cirrus level. The statistical analysis based on the 90 evaluated cloud cases revealed that all observed cloud layers with cloud top temperatures of below −15°C contained ice. Typically (under non‐volcanic aerosol conditions) such a high fraction of ice‐containing clouds is not reached before temperatures decrease below −25°C over central Europe.
Key Points
Lidar‐based observation of the interaction between volcanic ash and clouds
Case studies of ash‐affected heterogeneous freezing in free‐tropospheric clouds
Statistical evaluation of the ice nucleating efficiency of volcanic aerosol
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